Biodiesel is a non-toxic, biodegradable, renewable diesel fuel obtained from lipid feedstocks. The most common production process for biodiesel is through the batch transesterification of vegetable oils with methanol. In batch and some continuous processes, the concentrations at which the catalyst is used to drive the reaction to completion can result in high material and processing costs. Greater attention is being focused on glycerols being a potential cause of biodiesel instability. Reaction conditions can result in the formation of oxidized by-products that cause discoloration. This is especially when glycerols are exposed to either high temperature or alkaline pH. Glycerols can oxidize or thermally degrade to form diols or acids that can contribute to the instability of biodiesel. These degradation by-products may catalyze the polymerization of unsaturated fatty acids causing the formation of gels. In addition to this, high alkalinity is linked to the excessive formation of soaps and the degradation of components found in lipid feedstocks, which impart color to the resulting biodiesel.
Much work on heterogeneous catalysts has been done in the past decade; however most of the work was done at temperatures and pressures well above those used in atmospheric batch processes. Under these conditions, the reaction rates of homogeneously catalyzed reactions would be enhanced. However, equilibrium limitations require substantial amounts of catalyst be present to drive the reactions to completion at or near the normal boiling point of methanol, the most widely used alcohol in the transesterification reaction.
Purification of a fuel or fuel additive formed using a catalyst can be conducted to purify the product to remove or reduce undesirable compounds or degradation products such as diols. While distillation may be used for purification, it has the disadvantage that non-volatile free-radical scavengers, such as tocopherols (vitamin E and its derivatives) normally present in oils would not substantially be carried over in the distillate. Such compounds have the beneficial effect of scavenging free radicals that may promote degradation of the product. The product life of a fuel such as biodiesel may be expected to meet or exceed about 3 months, but without the presence of tocopherols, a product purified through distillation may have a greatly reduced product life (or “shelf life”). It is desirable to maintain such free radical scavengers as tocopherols in a biofuel to maintain as long of a shelf life as possible. The color of a biodiesel product versus that of the initial lipid is an indicator of the formation of degradation products, the darker the product, the higher the level of degradation products contained therein. Further, the higher a product's resistance to degradation through oxidation, the more commercially valued the product.
There is a need for a method of forming a biofuel that reduces the quantity of catalyst needed, while maximizing the presence of tocopherols and minimizing the presence of degradation products.